1. Field of the Invention
This invention is concerned with the field of optical recording systems in general. More particularly, it discloses a new Infrared Holographic recording method.
2. Prior Art
General interest in infrared holography has continued to grow in the last few years, despite the paucity of materials in this spectral region. Infrared holograms are known to be useful in infrared seeking missile systems, infrared communication system optics and infrared imaging for night vision image intensifiers.
Reviewing the current infrared materials, reported in literature, we find that the mechanisms of image formation generally fall into three classes: thermal, solid state, or photo-conductive effects.
Holograms have been recorded with IR radiation (0.75 to 1.5 .mu.m) in many diverse materials. Photochromic spiropirans (T. Izawa and M. Kamiyama, Appl. Phys. Letters 15 (1969), pp. 201-203) and liquid crystals (W. A. Simpson and W. E. Deeds, Applied Optics 9 (1970), pp. 499-501; and F. Keilmann, Appl. Optics 9 (1970), pp. 1319-1322) are capable of recording low resolution interferograms by thermal processes with 10.6 .mu.m CO.sub.2 laser illumination. Silicon crystals (J. P. Woerdman, Optics Communications 2 (1970), pp. 212-214) can record transient holograms with Q-switched Nd: YAG laser pulses by the creation of free carriers. The ingenious photoconductor-thermoplastic devices (W. S. Colburn, L. M. Ralston and J. C. Dwyer, Appl. Phys. Letters 23, (1973), pp. 145-146) show sensitivity beyond 1 .mu.m wavelengths and possess read-write-erase capability. Photographic emulsions (C. Roychoudhuri and B. J. Thompson, Optics Communications 10 (1974), pp. 23-25) can be dye-sensitized to 1.06 .mu.m radiation, but they lack resolution characteristic of visible light (0.4-0.7 .mu.m radiation) holographic plates. Dye sensitization of photopolymers (J. A. Jenney, J. Opt. Soc. Am. 60 (1970), pp. 115-1161) and dichromated gelatin (A. Graube, Optics Communications 8 (1973), pp. 251-253) have extended their spectral sensitivity to about 720 nm, but further infrared sensitivity is lacking.
All of the above materials, however, are unsatisfactory for recording holographic optical elements (HOE) when diffraction efficiency, spatial frequency, and permanence are considered.
The thermal effect materials typically suffer from a low resolution limit. This limit appears to result from the lateral conduction of heat through interference fringe boundries, and its extension beyond approximately 70 cycles/mm is highly improbable. Additionally, holograms formed in these materials have a very limited lifetime, and since the images are generally recorded as thick amplitude gratings, the diffraction efficiency is theoretically limited to approximately 3.7%. (J. C. Urbach, "Advances in Hologram Recording Materials," Developments in Holography, S.P.I.E. Seminar Proceedings, vol. 25, p. 17-41, April 14-15, 1971). Hence the thermal effect materials fall short in several catagories, and the possibility of substantially improving their characteristics is very remote.
In the solid state materials class silicon crystals have been included for the sake of completeness, but their application to HOE's is immediately preempted by the short persistence time (i.e., 25 nsec) of the holographic image. The photographic emulsion can be dye sensitized to respond to wavelength exceeding 1.1 microns. (C. E. K. Mees and T. H. James, The Theory of the Photographic Process, Macmillan Co., New York, 1966., Ch. 12). This long wavelength sensitization, however, is restricted to relatively large silver halide crystals, which severely limit the resolution of the film. The 4-Z plate is Kodak's newest near infrared product, but the spatial frequency is limited to less than 25 cycles/mm.
The thermoplastic material utilizes the photoconductive effect to initiate the image recording process. The actual image is recorded in a thermoplastic with the aid of a xerographic surface charge. (L. H. Lin and H. L. Beauchamp, Appl. Opt. 9 (1970) 2088-2092). Exposures with 1.15 micron radiation produce up to 5% diffraction efficiency, but the efficiency drops to 0.15% at a spatial frequency of 1000 cycles/mm. (W. S. Colburn, L. M. Ralston, and J. C. Dwyer, Appl. Phys. Letters 23 (1973) 145-146). Unfortunately, since the holograms are recorded as thin phase images, the theoretical maximum diffraction efficiency is limited to 33.9% (See Unbach Supra).
In seeking to overcome the limitations discussed above and bringing near infrared HOE's closer to reality, it was discovered that holographic images can be recorded in photographic emulsions by utilizing the Herschel effect.
In 1840, Sir John Herschel discovered that latent images recorded in photographic emulsions could be selectively erased by irradiation at infrared wavelengths (Sir J. F. W. Herschel, Philosophical Transactions (1840) pp. 1-59). This phenomenon is called the Hershel reversal effect. It has been applied to various photographic processes for making direct positive prints or copies. However, processes employing this principle required comparatively large illumination energies. These processes differed from that employed in the fabrication of holograms in that conventional light sources were used and generally nontransparent prints or copies were obtained.